The proposed research is to study gene regulation in Trypanosome cruzi, the protozoan parasite which causes Chagas' disease. The long-term objective is to gain an understanding of the molecular mechanisms responsible for regulating the stage-specific expression of the FL-160 surface antigen on T. Cruzi. FL-160 surface proteins are found only on the mammalian trypomastigote form but not the insect epimastigote stage. Understanding the regulation of stage-specific surface proteins will help in understanding the mechanisms of adaptation of the parasite to the mammalian host. Preliminary evidence, presented in this grant. suggests that FL-I 60 stage.specific regulation occurs as a post.transcriptional event, perhaps by differential RNA stability. Transient transfection experiments suggest that a cis-acting element in the 3 untranslated region (UTR) of the FL-160 gene is responsible for the stage-specific regulator. The Specific aims of this project are to : I) confirm that the cis-acting element in the 3' UTR is responsible for the stage-specific regulation using stable transfection; 2) map the cis-acting regulatory element within the 3' UTR; and 3) test for the presence and characterize trans- acting factors that participate in the regulatory mechanisms. In the future it is hoped that trans-acting genes can be identified that are responsible for regulating stage-specific expression of surface antigens. By altering the function of the trans-acting genes. it may be possible to alter the expression of these surface antigen genes to discover their role in infection and pathogenesis.